This invention relates to thick film circuits and in particular to a method which allows non-noble metal conductors to be included with thick film resistors.
At the present time, most thick film circuits employ a conductor system based on the noble metals silver, gold, palladium and platinum, alone or in combination, and ruthenium-based resistor material. Precious metal pastes, though providing high conductivity, are fairly costly. In addition, silver-based pastes can result in migration of silver across the substrate to cause short circuits. Furthermore, poor solderability often results noble-metal systems are used.
In view of the above problems, workers in the art have proposed alternatives to the noble-metal system and in particular the use of copper-based conductors. For example, copper pastes which are deposited and fired in a nitrogen atmosphere have been developed (see, e.g., U.S. Pat. No. 4,072,771 issued to Grier). However, such pastes are incompatible with ruthenium-based resistor materials which require firing in an oxidizing atmosphere.
Other proposals have been made which permit a copper paste conductor requiring a reducing atmosphere to be fabricated along with resistors requiring an oxidizing atmosphere. For example, we previously suggested depositing a fritless copper paste, firing at a high temperature, then reducing the material and re-oxidizing to form a low density copper oxide. The resistors were then deposited and fired in air, followed by reducing the copper oxide at a sufficiently low temperature so as not to degrade the resistors (see U.S. Pat. No. 4,140,817, issued to Brown et al). Although effective, the process required a high initial firing in a special furnace and extra process steps. Therefore, we later proposed using a fritted copper paste which could be fired at a low temperature. After the resistors were formed, the material could be reduced at a low temperature which did not significantly affect the resistors (see U.S. Pat. No. 4,316,920 issued to Brown et al). While such a process was satisfactory, a need still existed for improved conductivity, adhesion and solderability of the conductor material.
It is therefore a primary object of the invention to provide a method of fabricating a thick film circuit which includes a non-noble metal conductor requiring a reducing atmosphere and a resistor requiring an oxidizing atmosphere in a manner which results in improved conductivity, adhesion and solderability of the conductor.